The present invention relates to an FMCW radar linearizer and to an FMCW radar including such a linearizer.
FMCW radar linearizers are known per se. One example is disclosed by Peebles, P. Z. and Green, A. H., "On Performance of a Linear F. M. Radar Transmitter at 35 GHz", Proceedings of IEEE, Southcon, 1982. In summary, an artificial (or synthetic) target is produced using an rf time delay device to which the microwave rf signal is applied using an rf voltage controlled oscillator (VCO). The input signal to, and the output signal from, the time delay device are mixed together and the output signal is applied to a phase detector to which a reference oscillator is connected. Any shift in frequency from a constant value is detected and is used to produce a voltage proportional to the phase error. This voltage is amplified and fed back to the rf VCO with polarity reversal whereby it is used to reduce dynamically the original frequency error.
This article shows in FIG. 2 thereof a practical system in which, in order to avoid performance degradation due to direct leakage through the phase detector, two additional rf mixers are provided to which different local oscillator signals are provided. A first of these additional rf mixers is connected in the signal path between the rf VCO and the input to the time delay device and the second of these additional rf mixers is connected in the signal path to an input of the first mentioned mixer to which in the basic arrangement the undelayed rf VCO is applied. This refinement to the basic arrangement has been proposed in order to alleviate a phase detector leakage problem.
The time delay introduced in order to generate the artificial target is of the order of 20 ns. At frequencies of the order of 1.5 GHz such a time delay is normally produced by means of a coaxial cable which, at these frequencies, is very lossy, greater than 40dB. Such losses increase with increasing frequency.
A second order effect which results from generating an artificial target using a time delay of 20 ns is that it introduces high constraints on the linearizer. Accordingly the linearizer will not be operating in its optimum manner.
The drawbacks of this known linearizer are that it operates at microwave frequencies which means that the components such as mixers are costly, delay devices are lossy and high constraints are imposed on the linearizer.